Semiconductor laser device optical disk apparatus and optical integrated unit

ABSTRACT

A semiconductor laser device includes: a first cladding layer, which is made of a nitride semiconductor of a first conductivity type and is formed over a substrate; an active layer, which is made of another nitride semiconductor and is formed over the first cladding layer; and a second cladding layer, which is made of still another nitride semiconductor of a second conductivity type and is formed over the active layer. A spontaneous-emission-absorbing layer, which is made of yet another nitride semiconductor of the first conductivity type and has such an energy gap as absorbing spontaneous emission that has been radiated from the active layer, is formed between the substrate and the first cladding layer.

BACKGROUND OF THE INVENTION

The present invention generally relates to a semiconductor laser devicefor emitting blue laser radiation, which is applicable as a laser lightsource to an optical information storage medium such as an optical disk.More particularly, the present invention relates to a semiconductorlaser device that can prevent spontaneous emission from leaking out ofthe device and also relates to an optical disk apparatus and opticalintegrated unit using such a device.

A nitride semiconductor laser device, which is made of nitridesemiconductors containing Group III elements like aluminum (Al), gallium(Ga) and/or indium (In), emits blue laser radiation in a wavelengthrange from 400 to 500 nm when its active layer is made of an InGaNcompound. Currently, a red-light-emitting semiconductor laser device,which emits laser radiation in the red part of the spectrum with awavelength of 650 nm, is applied to recording or reproducing informationfrom a DVD. However, if such a blue-light-emitting semiconductor laserdevice is actually applied to a DVD, then the storage capacity of theDVD can be increased threefold or more. Specifically, the storagecapacity of a disk per side with a diameter of 12 cm can be raised to ashigh as 15 gigabytes or more. The implementation of suchblue-light-emitting semiconductor laser devices is in high demand. Thisis because a high definition (HD) video signal can be reproduced from adisk with a storage capacity of 15 gigabytes for as long as two hours ormore according to the MPEG-2 standard, an international standard ofmoving picture compression. A nitride semiconductor laser device, whichcan oscillate successfully at such a short wavelength (i.e., in therange from about 400 to about 450 nm), includes an InGaN active layerand AlGaN cladding layers.

The InGaN active layer and AlGaN cladding layers are usually formed bygrowing InGaN and AlGaN crystals over a substrate. Accordingly, a GaNsubstrate, which has the same crystal structure as InGaN and AlGaN and alattice constant almost equal to those of InGaN and AlGaN, is bestsuited to growing these layers with minimized crystal imperfections. Inthe current state of the art, however, a GaN substrate with a size of 10mm or more on each side is not available. Thus, a sapphire substrate(made of single crystalline Al₂O₃) is generally used as a substitutesubstrate. Although sapphire has a lattice constant that is differentfrom that of GaN by as much as 14%, sapphire is still appropriate forthe growth of nitride semiconductor crystals because sapphire has thesame crystal structure as GaN and is stable enough even at an elevatedtemperature of 1000° C. or more.

In semiconductor laser devices in general, not just in nitride ones,when current exceeding the threshold value thereof is injected into thep- and n-side electrodes thereof, electrons and holes, which have beencreated from the injected current, are recombined to emit radiation,which is strongly confined in the active layer. As a result, the currentis amplified, continuous oscillation occurs and the radiation is emittedas laser radiation through the emissive end facet of the active layer.

When the active layer of a nitride semiconductor laser device is made ofInGaN mixed crystals with an In mole fraction of 15%, bluish purplelaser radiation is obtained at an oscillation wavelength of 410 nm. Inthat case, a pair of cladding layers with a refractive index smallerthan that of the active layer should be provided over and under theactive layer, and the energy gap between the active layer and thecladding layers should be 0.4 eV or more. These cladding layers areusually made of AlGaN mixed crystals with an Al mole fraction of 7%.

Considering its principle of operation, however, the blue laserradiation emitted from the prior art nitride semiconductor laser deviceis harder to detect than the red laser radiation, and therefore thesignal-to-noise ratio (SNR) obtained at a light-receiving device such asa photodetector is usually low. Thus, compared to the red-light-emittingsemiconductor laser device, it is much more necessary for the nitridesemiconductor laser device of the short-wave-emission type to reduce thenoise produced from the device.

SUMMARY OF THE INVENTION

An object of the present invention is reducing noise, e.g., opticalnoise, in particular, that has been produced from a nitridesemiconductor laser device of the short-wave-emission type.

For the purpose of implementing a short-wave-emitting laser device, thepresent inventors looked into various imaginable sources of noise thatis produced from a blue-light-emitting laser device. As a result, wespotted the following optical factor as a root of the problem.

Specifically, on and after current is applied to a semiconductor laserdevice, spontaneous emission is created in the light-emitting region ofan active layer. And once the current applied has exceeded apredetermined value, the spontaneous emission is amplified in thelongitudinal direction of the resonant cavity, and then oscillationphenomenon starts to be observed. As a result, coherent laser radiationis emitted from the emissive end facet of the active layer. The currentat the predetermined value is called “oscillation threshold current”.After the applied current has reached the threshold value and before thelaser oscillation starts to be observed, only the spontaneous emissiongoes on increasing.

The spontaneous emission is not coherent laser radiation. Accordingly,if the spontaneous emission is mixed into the laser radiation, thenvarious adverse effects are caused. For example, where a laser device isused as an optical pickup for an optical disk, the spontaneous emissioncomponent constitutes noise for the laser radiation, thus decreasing theSNR when information is read out from the optical disk. Also, thespontaneous emission is emitted not just from the emissive end facet ofa laser chip. Accordingly, in an optical integrated unit in which alaser chip is integrated together with a photodetector or an electroniccircuit, the spontaneous emission becomes biasing light for thephotodetector to adversely affect the detection operation of thephotodetector.

To achieve the foregoing object, the present invention provides aspontaneous-emission-absorbing layer inside or on the surface of a laserchip to minimize the spontaneous emission leaking out of the chip. Also,the present invention provides a spontaneous emission protective filmshowing a high reflectance against the spontaneous emission on the upperor side faces of the chip such that the spontaneous emission does notleak out. In addition, the present invention provides means for blockingthe spontaneous emission between the laser chip and a monitoringphotodetector to prevent the spontaneous emission from being mixed intomonitoring light for use in automatic power control.

Hereinafter, specific means of the present invention for solving theproblem will be enumerated.

A first exemplary semiconductor laser device according to the presentinvention includes: a first cladding layer, which is made of a nitridesemiconductor of a first conductivity type and is formed over asubstrate; an active layer, which is made of another nitridesemiconductor and is formed over the first cladding layer; and a secondcladding layer, which is made of still another nitride semiconductor ofa second conductivity type and is formed over the active layer. Aspontaneous-emission-absorbing layer, which is made of yet anothernitride semiconductor of the first conductivity type and absorbsspontaneous emission that has been radiated from the active layer, isformed between the substrate and the first cladding layer.

In the first semiconductor laser device, the spontaneous emission thathas been radiated from the active layer toward the substrate is absorbedinto the spontaneous-emission-absorbing layer. That is to say, thespontaneous emission involved with the laser oscillation does not leakout of the device. Accordingly, optical noise, which adversely affectsoptical elements surrounding the device, can be greatly reduced.

In one embodiment of the present invention, thespontaneous-emission-absorbing layer may contain indium and be formed incontact with the first cladding layer. In such an embodiment, since thespontaneous-emission-absorbing layer contains indium, the energy gap ofthe spontaneous-emission-absorbing layer can be smaller than the energyof the spontaneous emission radiated from the nitride semiconductors. Asa result, the spontaneous emission can be absorbed just as intended.

In an alternate embodiment, the spontaneous-emission-absorbing layer maycontain indium and be formed in contact with the substrate. In such anembodiment, the spontaneous-emission-absorbing layer can also functionas a buffer layer to be formed on the upper surface of the substrate forimproving the crystallinity of nitride semiconductor layers growingthereon.

A second exemplary semiconductor laser device according to the presentinvention includes: a first cladding layer, which is made of a nitridesemiconductor of a first conductivity type and is formed over asubstrate; an active layer, which is made of another nitridesemiconductor and is formed over the first cladding layer; a secondcladding layer, which is made of still another nitride semiconductor ofa second conductivity type and is formed over the active layer; and anelectrode formed over the second cladding layer. Aspontaneous-emission-absorbing layer, which is made of yet anothernitride semiconductor of the second conductivity type and absorbsspontaneous emission that has been radiated from the active layer, isformed between the second cladding layer and the electrode.

In the second semiconductor laser device, the spontaneous emission thathas been radiated from the active layer toward the electrode locatedover the second cladding layer is absorbed into thespontaneous-emission-absorbing layer. Accordingly, optical noise, whichadversely affects optical elements surrounding the device, can begreatly reduced.

In one embodiment of the present invention, thespontaneous-emission-absorbing layer may contain indium and be formed incontact with the second cladding layer.

In an alternate embodiment, the spontaneous-emission-absorbing layer maycontain indium and be formed in contact with the electrode. In such anembodiment, the spontaneous-emission-absorbing layer can also functionas a contact layer to be formed over the second cladding layer in ohmiccontact with the electrode.

A third exemplary semiconductor laser device according to the presentinvention includes a laser diode body. The laser diode body includes: afirst cladding layer, which is made of a nitride semiconductor of afirst conductivity type and is formed over a substrate; an active layer,which is made of another nitride semiconductor and is formed over thefirst cladding layer; and a second cladding layer, which is made ofstill another nitride semiconductor of a second conductivity type and isformed over the active layer. A spontaneous emission protective film forabsorbing or reflecting spontaneous emission that has been radiated fromthe active layer is formed on a surface of the substrate, which surfaceis opposite to another surface thereof over which the active layer islocated.

In the third semiconductor laser device, the spontaneous emissionprotective film can prevent the spontaneous emission, which has beenemitted from the active layer toward the substrate, from leaking out.

A fourth exemplary semiconductor laser device according to the presentinvention includes a laser diode body. The laser diode body includes: afirst cladding layer, which is made of a nitride semiconductor of afirst conductivity type and is formed over a substrate; an active layer,which is made of another nitride semiconductor and is formed over thefirst cladding layer; and a second cladding layer, which is made ofstill another nitride semiconductor of a second conductivity type and isformed over the active layer. Spontaneous emission protective films forabsorbing or reflecting spontaneous emission that has been radiated fromthe active layer are formed on an emissive end facet of the laser diodebody, except for a portion of the active layer from which laserradiation is emitted, and on a reflective end facet of the laser diodebody, which faces the emissive end facet.

In the fourth semiconductor laser device, the spontaneous emissionprotective films can prevent the spontaneous emission, which has beenradiated from the active layer toward the electrode located over thesecond cladding layer, from leaking out. In this specification, thelaser diode (chip) body refers to a stack of multiple semiconductorlayers formed on a substrate, which includes a resonant cavity.

A fifth exemplary semiconductor laser device according to the presentinvention includes a laser diode body. The laser diode body includes: afirst cladding layer, which is made of a nitride semiconductor of afirst conductivity type and is formed over a substrate; an active layer,which is made of another nitride semiconductor and is formed over thefirst cladding layer; and a second cladding layer, which is made ofstill another nitride semiconductor of a second conductivity type and isformed over the active layer. Spontaneous emission protective films forabsorbing or reflecting spontaneous emission that has been radiated fromthe active layer are formed on side faces of the laser diode body, whichare parallel to a direction in which the laser radiation is emitted.

In the fifth semiconductor laser device, the spontaneous emissionprotective films can prevent the spontaneous emission, which has beenradiated from the active layer toward the side faces of the laser diodebody, from leaking out.

In the third through fifth semiconductor laser devices, the spontaneousemission protective film is preferably made of silicon or a metalcontaining gold. Since the energy gap of silicon is smaller than theenergy of the spontaneous emission radiated from the nitridesemiconductors, the spontaneous emission can be absorbed just asintended. In addition, gold is a highly stable material and can reflectthe spontaneous emission that has been radiated from the nitridesemiconductors at a high reflectance. Furthermore, gold and silicon arecompatible with a semiconductor device fabrication process very easily.

A sixth exemplary semiconductor laser device according to the presentinvention includes: a first cladding layer, which is made of a nitridesemiconductor of a first conductivity type and is formed over asubstrate; an active layer, which is made of another nitridesemiconductor and is formed over the first cladding layer; a secondcladding layer, which is made of still another nitride semiconductor ofa second conductivity type and is formed over the active layer; and anelectrode, which is formed over the second cladding layer and injectsstriped current into the active layer. A recess is formed in the activelayer beside and along a region of the active layer to which the stripedcurrent is injected. The recess is filled in with aspontaneous-emission-absorbing member for absorbing spontaneous emissionthat has been radiated from the active layer.

In the sixth semiconductor laser device, thespontaneous-emission-absorbing member that fills the recess providedalong the current injection area prevents the spontaneous emission,which has been radiated from the active layer toward the side faces ofthe laser diode body, from leaking out.

A seventh exemplary semiconductor laser device according to the presentinvention includes a laser diode body. The laser diode body includes: afirst cladding layer, which is made of a nitride semiconductor of afirst conductivity type and is formed over a substrate; an active layer,which is made of another nitride semiconductor and is formed over thefirst cladding layer; and a second cladding layer, which is made ofstill another nitride semiconductor of a second conductivity type and isformed over the active layer. A spontaneous emission protective memberfor absorbing or reflecting spontaneous emission that has been radiatedfrom the active layer is formed on or over a surface of the substrate,which is opposite to another surface thereof over which the active layeris located, and another spontaneous emission protective member is formedto be spaced apart from at least one side face of the laser diode body.

In the seventh semiconductor laser device, the spontaneous emissionprotective members prevent the spontaneous emission, which has beenradiated from the active layer toward the substrate and/or the sidefaces of the laser diode body, from leaking out.

An eighth exemplary semiconductor laser device according to the presentinvention includes: a semiconductor laser chip; a photoelectrictransducer for receiving laser radiation that has been emitted from thesemiconductor laser chip and detecting a value of optical output powerof the semiconductor laser chip; and means for blocking at least part ofspontaneous emission that has been radiated from the semiconductor laserchip. The blocking means is provided between a portion of thesemiconductor laser chip from which the laser radiation is emitted and aportion of the photoelectric transducer at which the laser radiation isreceived.

In the eighth semiconductor laser device, it is possible to reduce thequantity of the spontaneous emission that enters the photoelectrictransducer for monitoring the optical output power of the semiconductorlaser chip. Thus, automatic power control can be carried out just asintended.

In one embodiment of the present invention, the blocking means ispreferably provided such that the spontaneous emission attenuates at aratio higher than the laser radiation while passing through the blockingmeans.

In another embodiment of the present invention, the blocking means maybe a light-blocking plate with an opening, through which an optical axisof the laser radiation passes.

In still another embodiment, the semiconductor laser chip may include asubstrate made of a material transmitting the laser radiation.

In this particular embodiment, the semiconductor laser chip ispreferably made of a nitride semiconductor, and the substrate ispreferably made of sapphire, silicon carbide or gallium nitride.

An optical disk apparatus according to the present invention includes:one of the first through eighth semiconductor laser devices of thepresent invention; a condensing optical system for condensing laserradiation that has been emitted from the semiconductor laser device on astorage medium on which data has been recorded; and a photodetector fordetecting the laser radiation that has been reflected from the storagemedium.

An optical integrated unit according to the present invention includes,on a support member made of a semiconductor, one of the first througheighth semiconductor laser devices of the present invention and aphotodetector for detecting reflected part of laser radiation that hasbeen emitted from the semiconductor laser device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a structure of a nitridesemiconductor laser device according to a first embodiment of thepresent invention.

FIG. 2 is a cross-sectional view illustrating a structure of a nitridesemiconductor laser device according to a first modified example of thefirst embodiment.

FIG. 3 is a cross-sectional view illustrating a structure of a nitridesemiconductor laser device according to a second modified example of thefirst embodiment.

FIG. 4 is a cross-sectional view illustrating a structure of a nitridesemiconductor laser device according to a third modified example of thefirst embodiment.

FIG. 5 is a cross-sectional view illustrating a structure of a nitridesemiconductor laser device according to a second embodiment of thepresent invention.

FIG. 6 is a perspective view illustrating a nitride semiconductor laserdevice according to a first modified example of the second embodiment.

FIG. 7 is a cross-sectional view illustrating a structure of a nitridesemiconductor laser device according to a second modified example of thesecond embodiment.

FIG. 8 is a graph illustrating how the optical output power changes withthe injected current in inventive and conventional nitride semiconductorlaser devices.

FIG. 9 is a cross-sectional view illustrating a structure of a nitridesemiconductor laser device according to a third embodiment of thepresent invention.

FIG. 10 is a cross-sectional view illustrating a structure of a nitridesemiconductor laser device according to a fourth embodiment of thepresent invention.

FIGS. 11(a) and 11(b) are respectively cross-sectional view andperspective view illustrating a nitride semiconductor laser deviceaccording to a modified example of the fourth embodiment.

FIG. 12 schematically illustrates an arrangement of an optical diskapparatus according to a fifth embodiment of the present invention.

FIG. 13 is a perspective view schematically illustrating an opticalintegrated unit according to a sixth embodiment of the presentinvention.

FIG. 14 is a perspective view partly in section illustrating asemiconductor laser device according to a seventh embodiment of thepresent invention.

FIG. 15 schematically illustrates how to perform automatic power controlon the semiconductor laser device according to the seventh embodiment.

FIG. 16(a) is a graph illustrating an I-L characteristic between theoperating current and optical output power in an ordinary semiconductorlaser device; and

FIG. 16(b) is a graph illustrating how the I-L characteristics depend onthe temperature.

FIG. 17 is a graph illustrating, in comparison, respective I-Lcharacteristics of prior art short- and long-wave-oscillatingsemiconductor laser devices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

Hereinafter, a first embodiment of the present invention will bedescribed with reference to FIG. 1.

FIG. 1 illustrates a cross-sectional structure of a nitridesemiconductor laser device 1A according to the first embodiment. Asshown in FIG. 1, the nitride semiconductor laser device 1A is mountedfacedown over a submount 2 such that the circuitry side of the device 1Afaces the principal surface of the submount 2, which is made of siliconcarbide (SiC) or diamond with insulation properties. In thisspecification, the “nitride semiconductor laser device” refers to asemiconductor laser device made of compound semiconductors representedby a general formula Al_(x)Ga_(y)In₂N, where 0≦x≦1, 0≦y≦1, 0≦z≦1 andx+y+z=1).

In the nitride semiconductor laser device 1A, a buffer layer 12 of GaNor AlN is formed on the principal surface of a sapphire substrate 11,which faces the submount 2 (i.e., its circuitry side). The buffer layer12 is provided to buffer a misfit of lattice constants between sapphireand GaN crystals and thereby obtain semiconductor layers with reducedcrystal imperfections. On the buffer layer 12, a contact layer 13 ofn-type GaN doped with Si is formed. On the lower surface of the n-typecontact layer 13, an active region, on which a double heterojunction(DH) laser structure will be formed, and an n-type electrode region, onwhich an n-side electrode will be formed, are defined. As shown in FIG.1, an n-side electrode 14 is formed by alternately stacking Ti and Allayers or Mo, Pt and Au layers on the n-side electrode region.

On the active region in the lower surface of the n-type contact layer13, n-type In_(x)Ga_(1−x)N (where 0<x<1) spontaneous-emission-absorbinglayer 15A, n-type AlGaN cladding layer 16, In_(y)Ga_(1−y)N (where 0<y<1)active layer 17, p-type AlGaN cladding layer 18, p-type GaN contactlayer 19 and insulating film 20 of silicon dioxide with striped openingsare stacked in this order. The spontaneous-emission-absorbing layer 15Ahas an energy gap, which is small enough to absorb the spontaneousemission that has been radiated from the active layer 17. The n-typecladding layer 16 confines not only the radiation produced in the activelayer 17 but also electrons within the active layer 17, while the p-typecladding layer 18 confines not only the radiation produced in the activelayer 17 but also holes within the active layer 17. As a result, theelectrons and holes confined in the active layer 17 are recombinedtogether to create recombined radiation. The p-type contact layer 19makes ohmic contact with a p-side electrode 21. In the illustratedembodiment, the In mole fraction x in the spontaneous-emission-absorbinglayer 15A is equal to or larger than the In mole fraction y in theactive layer 17.

Supposing the In mole fraction y in well layers of the active layer 17to be 15%, the laser radiation emitted oscillates at a wavelength of 410nm. Accordingly, if the In mole fraction x in thespontaneous-emission-absorbing layer 15A is 15% or more, then thespontaneous-emission-absorbing layer 15A is no longer transparent to thespontaneous emission that has been radiated from the active layer 17.Thus, the spontaneous-emission-absorbing layer 15A can absorb thespontaneous emission not contributing to the laser oscillation. In theillustrated embodiment, the thickness of each well layer is supposed tobe about 5 nm or less.

The p-side electrode 21 is formed on the lower surface of the insulatingfilm 20 by alternately stacking Ni and Au layers or Ni, Pt and Au layersthereon, for example, so as to fill in the striped openings of theinsulating film 20.

On the principal surface of the submount 2, p- and n-side terminalelectrodes 23 and 25 are formed to face the p- and n-side electrodes 21and 14, respectively. The p- and n-side electrodes 21 and 14 areelectrically connected to the p- and n-side terminal electrodes 23 and25, respectively, with solder members 22 and 24 containing lead (Pb) andtin (Sn) or gold (Au).

As described above, even if current exceeding the threshold valuethereof is being injected into the p- and n-side electrodes 21 and 14 ofthe nitride semiconductor laser device 1A, spontaneous emission, notcontributing to laser oscillation, is continuously radiated from theactive layer 17. Also, the spontaneous emission at a wavelength of about410 nm is transmissible through any of the sapphire substrate 11, n- andp-type AlGaN cladding layers 16, 18 and n- and p-type GaN contact layers13, 19. Thus, the spontaneous emission is radiated in all the directionsaround the active layer 17.

According to this embodiment, however, thespontaneous-emission-absorbing layer 15A with an In mole fraction largerthan that of the active layer 17, i.e., with an energy gap smaller thanthat of the active layer 17, is provided between the n-type claddinglayer 16 and n-type contact layer 13. That is to say, thespontaneous-emission-absorbing layer 15A is not transparent to thespontaneous emission that has been radiated from the active layer 17. Asa result, the spontaneous emission is absorbed into thespontaneous-emission-absorbing layer 15A.

As can be seen, after the nitride semiconductor laser device 1A has beenmounted on the submount 2, it is possible to substantially prevent theleakage of the spontaneous emission radiated from the active layer 17toward the substrate 11. Thus, according to this embodiment, opticalnoise, which adversely affects optical elements around the substrate 11,can be greatly reduced.

Also, since the spontaneous-emission-absorbing layer 15A is provided incontact with the n-type cladding layer 16, the layer 15A is closest tothe active layer 17 functioning as the light source. Accordingly, thespontaneous emission can be absorbed at a high efficiency.

In the foregoing embodiment, the spontaneous-emission-absorbing layer15A is made of InGaN with an energy gap smaller than that of the activelayer 17. Alternatively, the spontaneous-emission-absorbing layer 15Amay be made of any other semiconductor compound so long as GaNsemiconductor crystals can grow thereon.

Hereinafter, a method for fabricating the nitride semiconductor laserdevice 1A with such a structure will be described.

In the illustrated embodiment, a metalorganic vapor phase epitaxy(MOVPE) process is carried out to grow the nitride semiconductorcrystals. Specifically, organometallic compound source materials such astrimethylaluminum (TMA), trimethylgallium (TMG) and trimethylindium(TMI) are introduced as Group III element sources into a reactor at anelevated temperature. In addition, ammonium (NH₃) gas is also introducedas a nitrogen source into the reactor, and the organometallic compoundsource materials and NH₃ gas are thermally decomposed within thereactor, thereby growing nitride semiconductor crystals on a substrate.

More specifically, first, the temperature of the substrate 11 placedwithin the reactor is set at about 600° C. Then, the GaN buffer layer 12is grown on the principal surface of the substrate 11 with TMG and NH₃introduced into the reactor. It is known that various defects such ascracking are less likely to be caused by providing such a“low-temperature buffer layer” on sapphire with a different latticeconstant from those of GaN semiconductors. To grow an AlN buffer layer12 on the substrate 11, TMG should be replaced with TMA.

Next, the temperature of the substrate 11 is raised to about 1000° C.,and the n-type GaN contact layer 13 doped with Si as an n-type dopant isgrown on the upper surface of the buffer layer 12 with TMG and NH₃introduced into the reactor.

Subsequently, the temperature of the substrate 11 is lowered to about800° C., and then the n-type In_(x)Ga_(1−x)Nspontaneous-emission-absorbing layer 15A is grown on the upper surfaceof the n-type contact layer 13 with TMI additionally supplied as anotherGroup III element source.

Thereafter, the temperature of the substrate 11 is raised to about 1000°C. again, and then the n-type AlGaN cladding layer 16 is grown on theupper surface of the spontaneous-emission-absorbing layer 15A with TMGand TMA supplied as Group III element sources onto the substrate 11.

Next, the temperature of the substrate 11 is lowered to about 800° C.again, and then the In_(y)Ga_(1−y)N active layer 17 is grown on theupper surface of the n-type cladding layer 16 with the supply of TMAsuspended and TMI supplied as an alternative Group III element source.For example, the active layer 17 may be a multiple quantum well activelayer consisting of InGaN well layers and AlGaN barrier layers (each ofwhich is 5 nm thick or less) alternately stacked. In that case, laseroscillation starts at a smaller threshold current.

Thereafter, the temperature of the substrate 11 is raised to about 1000°C. again and then the p-type AlGaN cladding layer 18 doped with Mg as ap-type dopant is grown on the upper surface of the active layer 17 withTMG and TMA supplied as Group III element sources onto the substrate 11.Then, the p-type GaN contact layer 19 is grown on the upper surface ofthe p-type cladding layer 18 with the supply of TMA suspended.

Next, the substrate on which these nitride semiconductor layers havebeen stacked one upon the other (i.e., an epitaxial substrate) is takenout of the reactor and the insulating film 20 of silicon dioxide, forexample, is deposited on the upper surface of the p-type contact layer19. Subsequently, the insulating film 20 is selectively etched, therebyforming striped openings in the insulating film 20.

Then, the p-side electrode 21 is formed as a stack of Ni and Au layers,for example, over the entire surface of the insulating film 20, as wellas inside the openings, by an evaporation technique, for instance.

Next, part of the p-side electrode 21 in the active region is masked andthe epitaxial layers are etched sequentially, thereby partially exposingthe n-type contact layer 13 and defining the n-side electrode region.And the n-side electrode 14 is formed as a stack of Ti and Al layers,for example, by an evaporation technique, for instance.

Subsequently, the epitaxial substrate is cleaved to obtain a resonantcavity and the end facets of the cavity are coated with a predeterminedmaterial. Then, the substrate 11 of the nitride semiconductor laserdevice 1A is placed over the submount 2 such that the circuitry side ofthe substrate 11 faces the principal surface of the submount 2. And thep- and n-side electrodes 21, 14 are aligned with the p- and n-sideterminal electrodes 23, 25, respectively. Thereafter, these electrodes21, 23 and 14, 25 are bonded together with the solder 22 and 24,respectively. As a result, the nitride semiconductor laser device 1Ashown in FIG. 1 is completed.

It should be noted that n- and p-type GaN optical guide layers may beinserted between the active layer 17 and the n-type cladding layer 16and between the active layer 17 and the p-type cladding layer 18,respectively. In such a case, the radiation emitted can be confined inthe active layer 17 even more efficiently and the threshold current canbe further reduced.

Modified Example 1 of Embodiment 1

Hereinafter, a first modified example of the first embodiment of thepresent invention will be described with reference to FIG. 2.

FIG. 2 illustrates a cross-sectional structure of a nitridesemiconductor laser device 1B according to a first modified example ofthe first embodiment. In FIG. 2, the same members as those illustratedin FIG. 1 are identified by the same reference numerals and thedescription thereof will be omitted herein. As shown in FIG. 2, ann-type InGaN spontaneous-emission-absorbing layer 15B with an energy gapsmaller than that of the active layer 17 is formed between the bufferlayer 12 and the n-type contact layer 13 in the nitride semiconductorlaser device 1B according to this modified example.

In such a structure, the spontaneous-emission-absorbing layer 15B canentirely cover the principal surface of the substrate 11 and the leakageof the spontaneous emission, which has been radiated toward thesubstrate 11, can be prevented with much more certainty.

Also, if the spontaneous-emission-absorbing layer 15B is grown at a lowtemperature, then the layer 15B may be part of the buffer layer 12 anddoes not constitute an excessive burden on the fabrication process.

Modified Example 2 of Embodiment 1

Hereinafter, a second modified example of the first embodiment of thepresent invention will be described with reference to FIG. 3.

FIG. 3 illustrates a cross-sectional structure of a nitridesemiconductor laser device 1C according to a second modified example ofthe first embodiment. In FIG. 3, the same members as those illustratedin FIG. 1 are identified by the same reference numerals and thedescription thereof will be omitted herein. As shown in FIG. 3, a p-typeInGaN spontaneous-emission-absorbing layer 15C with an energy gapsmaller than that of the active layer 17 is formed between the p-typecladding layer 18 and the p-type contact layer 19 in the nitridesemiconductor laser device 1C according to this modified example. Inthis case, the spontaneous-emission-absorbing layer 15C should be ofp-type and Mg is used as a p-type dopant.

In such a structure, the spontaneous-emission-absorbing layer 15C cansubstantially prevent the leakage of the spontaneous emission that hasbeen radiated from the active layer 17 toward the p-side electrode 21.

Modified Example 3 of Embodiment 1

Hereinafter, a third modified example of the first embodiment of thepresent invention will be described with reference to FIG. 4.

FIG. 4 illustrates a cross-sectional structure of a nitridesemiconductor laser device 1D according to a third modified example ofthe first embodiment. In FIG. 4, the same members as those illustratedin FIG. 1 are identified by the same reference numerals and thedescription thereof will be omitted herein. As shown in FIG. 4, a p-typeInGaN spontaneous-emission-absorbing layer 15D with an energy gapsmaller than that of the active layer 17 is formed between the p-typecontact layer 19 and the p-side electrode 21 in the nitridesemiconductor laser device 1D according to this modified example.

In such a structure, the spontaneous-emission-absorbing layer 15D has asmaller energy gap, and therefore a lower contact resistance, than thep-type GaN contact layer 19. As a result, the threshold voltage can bereduced.

As described above, any of the spontaneous-emission-absorbing layers 15Athrough 15D is provided within the epitaxial layers of the laserstructure according to the first embodiment and its modified examples.The closer to the active layer 17 the spontaneous-emission-absorbinglayer is, the greater proportion of the laser radiation isunintentionally absorbed into the spontaneous-emission-absorbing layer,thus increasing the loss during laser oscillation. As a result, theoscillation threshold current increases to the contrary. Accordingly,the laser structure should be designed while carefully considering thedistribution of laser radiation.

Embodiment 2

Hereinafter, a second embodiment of the present invention will bedescribed with reference to FIG. 5.

FIG. 5 illustrates a cross-sectional structure of a nitridesemiconductor laser device 1E according to the second embodiment. InFIG. 5, the same members as those illustrated in FIG. 1 are identifiedby the same reference numerals and the description thereof will beomitted herein. According to the second embodiment, thespontaneous-emission-absorbing layer is provided on an outer surface ofthe laser chip body, not in the epitaxial layers as in the firstembodiment.

As shown in FIG. 5, a spontaneous emission protective film 31A forabsorbing or reflecting the spontaneous emission radiated from theactive layer 17 is formed on a surface of the sapphire substrate 11opposite to its circuitry side, i.e., its backside, in the nitridesemiconductor laser device 1E according to the second embodiment.

In the illustrated embodiment, crystalline or amorphous silicon with anenergy gap smaller than that of the spontaneous emission is used as amaterial for absorbing the spontaneous emission radiated from the InGaNactive layer 17. This silicon (Si) film may be formed by sputtering, forexample. Alternatively, any material other than Si may be used so longas the material has an energy gap smaller than the energy of thespontaneous emission and is easily compatible with the process.

The film 31A may also be made of a material strongly reflecting thespontaneous emission radiated from the active layer 17. For example, thefilm 31A may be an Au film or a stack of Au and other metal films, e.g.,a stack of Ti, Pt and Au films. The metal film or the multilayerstructure may be formed by an electron beam evaporation or resistanceheating evaporation technique.

Examples of the materials for the spontaneous emission protective film31A include not only Au, Ti and Pt, but also Cr, Sn, Cu, Fe, Ag and In.The spontaneous emission protective film 31A may be a single-layeredfilm of any of these metals, a multilayer structure consisting of atleast two of these metal films or a film made of an alloy containing atleast two of these metals (e.g., Au and Sn).

Furthermore, the spontaneous emission protective film 31A may be acombination of an absorptive film and a high-reflectance film. Forexample, even more remarkable effects are attainable if ahigh-reflectance film of Au is deposited on an absorptive film of Si.

As can be seen, after the nitride semiconductor laser device 1E has beenmounted on the submount 2, it is possible to substantially prevent theleakage of the spontaneous emission that has been radiated from theactive layer 17 toward the substrate 11. Thus, according to thisembodiment, optical noise, which adversely affects optical elementsdisposed around the substrate 11, can be greatly reduced in the nitridesemiconductor laser device 1E.

Modified Example 1 of Embodiment 2

Hereinafter, a first modified example of the second embodiment of thepresent invention will be described with reference to FIG. 6.

FIG. 6 illustrates the appearance of a nitride semiconductor laserdevice 1F according to a first modified example of the secondembodiment. In FIG. 6, the same member as that illustrated in FIG. 1 isidentified by the same reference numeral and the description thereofwill be omitted herein. As shown in FIG. 6, spontaneous emissionprotective films 31B for absorbing or reflecting spontaneous emissionradiated from the active layer are formed on an emissive end facet ofthe laser chip body, except for a portion of the active layer from whichlaser radiation is emitted, and on a reflective end facet facing theemissive end facet, in the nitride semiconductor laser device 1Faccording to this modified example.

The size of that portion of the active layer, from which the laserradiation is emitted, is about 5 μm square. Also, unless monitoringlaser radiation is not emitted through the reflective end facet, forexample, the spontaneous emission protective film 31B is preferablyformed over the entire surface of the reflective end facet. In such acase, the threshold value of oscillation can be further reduced.

The absorptive film is preferably an Si film as in the secondembodiment. The high-reflectance film may be the metal film exemplifiedabove. However, when a conductive film made of a metal, for example, isprovided, an insulating film of SiO₂ or the like should be formed inadvance as an undercoat so as not to cause electrical shortcircuit amongthe semiconductor layers.

In this modified example, the spontaneous emission protective films 31Bare provided on the end facets of the laser chip body, which cross atright angles with the direction in which the laser radiation resonates.Thus, these spontaneous emission protective films 31B should bereflective films rather than absorptive films. This is because thespontaneous emission that has been reflected from the spontaneousemission protective films 31B can contribute to laser oscillation.

The high-reflectance film may be a stack of dielectric films, e.g., SiO₂and TiO₂ films. In such a case, if SiO₂ and TiO₂ films, each beingone-fourth as thick as the wavelength of the spontaneous emission, arestacked alternately three times, then a reflectance up to 94% isattainable.

As can be seen, the leakage of the spontaneous emission, which has beenradiated from the active layer 17 in the direction approximatelyparallel to the longitudinal direction of the resonant cavity, can besubstantially prevented in this modified example. Thus, optical noisethat adversely affects optical elements placed in the direction in whichthe laser radiation is emitted from the nitride semiconductor laserdevice 1F and in the direction opposite to the former direction can begreatly reduced.

Modified Example 2 of Embodiment 2

Hereinafter, a second modified example of the second embodiment of thepresent invention will be described with reference to FIG. 7.

FIG. 7 illustrates a cross-sectional structure of a nitridesemiconductor laser device 1G according to a second modified example ofthe second embodiment. In FIG. 7, the same members as those illustratedin FIG. 1 are identified by the same reference numerals and thedescription thereof will be omitted herein. As shown in FIG. 7,spontaneous emission protective films 31C for absorbing or reflectingthe spontaneous emission radiated from the active layer 17 are formed onside faces of the laser chip body that are parallel to the direction inwhich the laser radiation is emitted in the nitride semiconductor laserdevice 1G according to this modified example.

The absorptive film is preferably an Si film as in the secondembodiment. The high-reflectance film may be the metal film exemplifiedabove (e.g., Au film). However, when a conductive film made of a metal,for example, is provided, an insulating film of SiO₂ or the like shouldbe formed in advance as an undercoat so as not to cause electricalshortcircuit among the semiconductor layers.

Furthermore, each of the spontaneous emission protective films 31C maybe a combination of absorptive and high-reflectance films. For example,a high-reflectance film of Au may be deposited on an absorptive film ofSi.

As can be seen, the leakage of the spontaneous emission, which has beenradiated from the active layer 17 in a direction approximately verticalto the longitudinal direction of the resonant cavity, can besubstantially prevented in this modified example. Thus, optical noisethat adversely affects optical elements placed in the direction verticalto the longitudinal direction of the resonant cavity can be greatlyreduced.

If the first and second embodiments of the present invention and theirmodified examples are adopted in combination, then the leakage of thespontaneous emission out of the laser chip body can be eliminated almostcompletely.

In order not to get the fabrication process too complicated, thespontaneous-emission-absorbing layer 15A of the first embodiment shownin FIG. 1 or the layer 15B of the first modified example thereof shownin FIG. 2 and the spontaneous emission protective film 31A of the secondembodiment shown in FIG. 5 may be provided as absorptive andhigh-reflectance films for a single semiconductor laser device.

FIG. 8 illustrates how the optical output power changes with theinjected current in an inventive nitride semiconductor laser device,which is provided with the spontaneous-emission-absorbing layer 15A andthe spontaneous emission protective film 31A, and a prior art nitridesemiconductor laser device. In FIG. 8, the axis of abscissas representsthe current (mA) injected into the laser device, while the axis ofordinates represents the output power (mW) of the laser radiation. Thecharacteristic curves 3 and 4 correspond to the inventive and prior artlaser devices, respectively. As can be seen from the curve 4 shown inFIG. 8, leakage of spontaneous emission is observed in the prior artsemiconductor laser device until the current reaches the threshold valueof oscillation. In contrast, as represented by the curve 3, almost nospontaneous emission is output and only the laser radiation is emittedin the vicinity of the threshold value of oscillation in the inventivesemiconductor laser device.

Embodiment 3

Hereinafter, a third embodiment of the present invention will bedescribed with reference to FIG. 9.

FIG. 9 illustrates a cross-sectional structure of a nitridesemiconductor laser device 1H according to a third embodiment of thepresent invention. In FIG. 9, the same members as those illustrated inFIG. 5 are identified by the same reference numerals and the descriptionthereof will be omitted herein. As shown in FIG. 9, a recess 26 isformed by etching beside a region (i.e., so-called “striped region”) inwhich current is injected from the p-side electrode 21 of the laser chipbody in the nitride semiconductor laser device 1H according to the thirdembodiment. The recess 26 vertically extends from the p-type contactlayer 19 to the n-type contact layer 13 to divide the active layer 17into two in the longitudinal direction of the resonant cavity. Therecess 26 horizontally extends substantially in parallel to the currentinjection region.

In addition, the spontaneous emission protective film 31A of silicon,for example, is formed on a surface of the substrate 11 opposite to itscircuitry side.

An insulating film 27 of silicon dioxide, for example, is deposited onthe inner wall of the recess 26. And the recess 26 is filled in with aspontaneous-emission-absorbing member 28 that can absorb the spontaneousemission, e.g., gold (Au). The spontaneous-emission-absorbing member 28is formed on the insulating film 27 by an evaporation technique, forexample.

The total thickness of the laser chip body, which is implemented as amultilayer structure formed by the MOVPE process, is as small as about 5μm. However, since the laser chip body includes multiple semiconductorlayers stacked one upon the other, the spontaneous emission that isradiated from these semiconductor layers in the direction parallel tothe surface of the substrate is non-negligible.

According to this embodiment, a spontaneous-emission-absorbing region isdefined by the recess 26 filled in with thespontaneous-emission-absorbing member 28 to divide the active layer 17into two beside the current injection region of the laser chip body.Thus, the spontaneous emission that is radiated in the directionparallel to the surface of the substrate 11 can be absorbed into thespontaneous-emission-absorbing region.

In the illustrated embodiment, the spontaneous-emission-absorbing member28 is made of Au. Alternatively, the spontaneous-emission-absorbingmember 28 may also be made of any other metal like Ti, Cr, Sn, Cu, Fe,Ag, Pt or In. The spontaneous-emission-absorbing member 28 may also be amultilayer structure consisting of at least two of these metal films(e.g., Ti and Au films) or a film made of an alloy containing at leasttwo of these metals. Also, the spontaneous-emission-absorbing member 28may be made of a semiconductor with a relatively small energy gap, e.g.,single crystalline or amorphous silicon.

In the third embodiment, the spontaneous-emission-absorbing member 28 isprovided beside the current injection region of the laser chip body onthe side opposite to the n-side electrode 14, i.e., on the left-handside of FIG. 9. Alternatively, the spontaneous-emission-absorbing member28 may be provided on the same side as the n-side electrode 14, i.e., onthe right-hand side of FIG. 9.

It is noted that even if no recess 26 is provided beside the currentinjection region on the same side as the n-side electrode 14 as shown inFIG. 9, the spontaneous emission is still absorbable into the soldermember 24 interposed between the n-side electrode 14 and terminalelectrode 25. Also, the solder member 22 containing Pb and Sn may bereplaced with a member made of a spontaneous-emission-absorbingmaterial, e.g., conductive adhesive or silver paste.

Moreover, the recess 26 has only to absorb the spontaneous emission thatis radiated from the active layer 17 in various directions other thanthe direction in which the waveguide extends. Thus, the recess 26 doesnot always have to extend in parallel to the current injection region.

As for the optical output power changing with the injected current, weconfirmed that the nitride semiconductor laser device according to thethird embodiment exhibits a similar characteristic to that representedby the curve 3 in FIG. 8.

As can be seen, the spontaneous-emission-absorbing member 28 providedinside the laser chip body and the spontaneous emission protective film31A provided on the backside of the substrate 11 opposite to itscircuitry side can reduce the leakage of the spontaneous emissionradiated from the laser chip. Thus, coherent laser radiation can beobtained according to this embodiment just as intended. Also, since aphotodiode used for monitoring the optical output power is less affectedby the spontaneous emission, the output power of the laser radiation iscontrollable more precisely.

Embodiment 4

Hereinafter, a fourth embodiment of the present invention will bedescribed with reference to FIG. 10.

FIG. 10 illustrates a cross-sectional structure of a nitridesemiconductor laser device 1I according to a fourth embodiment of thepresent invention. In FIG. 10, the same members as those illustrated inFIG. 5 are identified by the same reference numerals and the descriptionthereof will be omitted herein. As shown in FIG. 10, spontaneousemission protective walls 32 for absorbing or reflecting the spontaneousemission radiated from the active layer 17 are provided for the nitridesemiconductor laser device 1I according to this embodiment. Thespontaneous emission protective walls 32 are provided over the submount2 with the p- and n-side terminal electrodes 23 and 25 interposedtherebetween and are spaced apart from the side faces of the laser chipbody that are parallel to the direction in which the laser radiation isemitted.

In addition, the spontaneous emission protective film 31A of Au or Si,for example, is formed on the backside of the substrate 11 opposite toits circuitry side as in the second embodiment.

The spontaneous emission protective walls 32 may be made of the samematerial as that of the spontaneous emission protective film 31A. Amaterial that can absorb the spontaneous emission radiated from thelaser chip is preferably used.

The spontaneous emission protective walls 32 may be formed in thefollowing manner. First, block members of the metal exemplified above orSiC are prepared as bulks or sintered compacts. Then, the block membersare bonded onto the p- and n-side terminal electrodes 23, 25 of thesubmount 2 with solder, for example, so as to be spaced apart from theside faces of the laser chip. An insulating film may be interposedbetween the p- or n-side terminal electrode 23 or 25 and the spontaneousemission protective wall 32.

Alternatively, a thin film of the metal exemplified above may bedeposited by evaporation on the surface of a block member made of glassand the coated block member may be bonded onto each terminal electrode23, 25 with an adhesive, for example.

As can be seen, the spontaneous emission protective walls 32 providedfor the laser chip body so as to be spaced apart from the side facesthereof parallel to the direction in which the laser radiation isemitted and the spontaneous emission protective film 31A provided on thebackside of the substrate 11 opposite to its circuitry side can reducethe leakage of the spontaneous emission radiated from the laser chip.Thus, coherent laser radiation can be obtained according to thisembodiment just as intended.

Also, both the spontaneous emission protective film 31A and walls 32according to this embodiment are less likely to cause damage to theepitaxial layers of the laser chip.

It should be noted that only one spontaneous emission protective wall 32may be provided for a side face of the laser chip body in parallel tothe direction in which the laser radiation is emitted.

Furthermore, if the substrate 11 is not made of sapphire with insulationproperties but made of a conductor, e.g., n-type GaN, then thespontaneous emission protective film 31A may be used as the n-sideelectrode 14.

Modified Example of Embodiment 4

Hereinafter, a modified example of the fourth embodiment of the presentinvention will be described with reference to FIGS. 11(a) and 11(b).

FIGS. 11(a) and 11(b) respectively illustrate cross-sectional structureand appearance of a nitride semiconductor laser device 1J according to amodified example of the fourth embodiment. In FIGS. 11(a) and 11(b), thesame members as those illustrated in FIG. 10 are identified by the samereference numerals. As shown in FIGS. 11(a) and 11(b), a cover-likelight-blocking case 33 made of a metal containing Au, Ti or Pt isprovided over the submount 2 so as to cover the laser chip body entirelyin the nitride semiconductor laser device 1J according to this modifiedexample. The bottom of the light-blocking case 33 is spaced apart fromthe backside of the substrate 11 opposite to its circuitry side, whilethe inner walls thereof are spaced apart from the side faces of thelaser chip body. Also, the light-blocking case 33 is bonded onto the p-and n-side terminal electrodes 23 and 25 with an insulating film 34interposed therebetween.

Moreover, as shown in FIG. 11(b), the light-blocking case 33 is providedwith an opening 33 a for outputting the laser radiation therethrough.The opening 33 a is so located as to face a part of the laser chip fromwhich the laser radiation is emitted. Since the width of the currentinjection region of the active layer 17 is about 5 μm, the spot diameterof the laser radiation at the emissive end facet is approximately 5 μmsquare. Thus, the size of the opening 33 a may be equal to or slightlylarger than 5 μm.

Furthermore, if the output power of the laser radiation should bemonitored, then another opening (not shown) of the same shape as theopening 33 a may be provided for another side face of the light-blockingcase 33 so as to face the opening 33 a.

Also, depending on the intended application of the nitride semiconductorlaser device 1J, not all the four sides of the laser chip body have tobe covered with the case 33. For example, at least one, or both, of theside plates of the light-blocking case 33, which cross the optical axisof the laser radiation at right angles, may be omitted.

As for the optical output power changing with the injected current, weconfirmed that the nitride semiconductor laser device according to thefourth embodiment or its modified example exhibits a similarcharacteristic to that represented by the curve 3 in FIG. 8.

In all of the nitride semiconductor laser devices 1A through 1Jdescribed above, the p-type contact layer 19 is covered with theinsulating film 20 with striped openings, thereby defining stripedcurrent injection regions in the active layer 17. Alternatively, toimprove the controllability of the laser radiation in transverse mode,the p-type contact layer 19 and cladding layer 18 may be formed in aridge shape.

It should be noted that the present invention is applicable not only tothe nitride semiconductor laser device including the InGaN active layer17, but also to a nitride semiconductor laser device including asubstrate 11 with an energy gap larger than that of the active layer 17.

In the foregoing embodiments, nitride semiconductor crystals are grownby the MOVPE process. Alternatively, the crystals may be grown by amolecular beam epitaxy (MBE) process or a halide vapor phase epitaxy(H-VPE) process, for example.

Embodiment 5

Hereinafter, a fifth embodiment of the present invention will bedescribed with reference to FIG. 12.

FIG. 12 schematically illustrates an arrangement of an optical diskapparatus according to the fifth embodiment. The optical disk apparatusof the fifth embodiment uses the inventive nitride semiconductor laserdevice 41 as a light source thereof. As shown in FIG. 12, thesemiconductor laser device 41 is disposed at such a position that theemissive end facet thereof faces the data-retaining side of an opticaldisk 50, i.e., a storage medium on which desired data has been recorded.And a condensing optical system 40 is provided between the semiconductorlaser device 41 and the optical disk 50 in this optical disk apparatus.

The condensing optical system 40 includes collimator lens 42,diffraction grating 43, half prism 44 and condenser lens 45, which areplaced in this order such that the collimator lens 42 is closest to thesemiconductor laser device 41. The collimator lens 42 collimatesoutgoing radiation 51, which has been emitted from the semiconductorlaser device 41, into parallel light. The diffraction grating 43 splitsthe parallel light into three beams (not shown). The half prism 44transmits the outgoing radiation 51 and changes the optical path oflight 52 that has been reflected from the optical disk 50. And thecondenser lens 45 condenses the three beams onto the optical disk 50. Inthe illustrated embodiment, laser radiation with a wavelength of about410 nm is used as the outgoing radiation 51.

Each of the three beams is condensed on the optical disk 50 as a spotwith a diameter of about 0.4 μm. An optical disk drive 46 is furtherprovided to correct a radial focus error, which is detected from theoptical disk 50 based on these three spots, by moving the condenser lens45 over an appropriate distance.

On the optical path of the reflected light 52 outgoing from the halfprism 44, provided are a receiving lens 47 for converging the reflectedlight 52, a cylindrical lens 48 for detecting a focus error, and aphotodiode 49 for converting the condensed reflected light 52 intoelectrical signals.

As described above, the optical disk apparatus includes the condensingoptical system 40 for guiding the outgoing radiation 51 emitted from thesemiconductor laser device 41 onto the optical disk 50 and thephotodiode 49 receiving the light 52 that has been reflected from theoptical disk 50. And this apparatus further includes the semiconductorlaser device 41 as a light source that can minimize or substantiallyprevent the leakage of spontaneous emission. Thus, even if data has beenrecorded on the optical disk 50 at a high density, the optical diskapparatus can read out data from the disk 50 with an increased SNR. As aresult, this apparatus can perform a readout (reproduction) operationwith lower distortion.

In recording data on the optical disk 50, the output power of the laserradiation should be controlled precisely over a wide range from as lowas about 5 mW to as high as about 30 mW. This is because tracking shouldbe performed with low output power responsive to an address specifiedbefore data is written onto the disk 50 at 30 mW.

The optical disk apparatus according to the fifth embodiment cansubstantially prevent the spontaneous emission, which will be anon-negligible source of noise, from leaking out of the semiconductorlaser device 41. Thus, a monitoring photodiode (not shown) that monitorsthe optical output power of the semiconductor laser device 41 canconvert the output power of the laser radiation into current moreaccurately. Specifically, when the optical output power of thesemiconductor laser device 41 is changed, the monitor current changeslinearly in that situation. Accordingly, the output operation of thesemiconductor laser device 41 can be controlled far more precisely andthe performance of the optical disk apparatus is enhanced during bothrecording and reproducing operations.

Embodiment 6

Hereinafter, a sixth embodiment of the present invention will bedescribed with reference to FIG. 13.

FIG. 13 schematically illustrates a configuration for an opticalintegrated unit according to the sixth embodiment. As shown in FIG. 13,the optical integrated unit of this embodiment is formed on a singlesupport member 61 of Si. A concave portion 61 a is provided in theprincipal surface of the support member 61, and the inventivesemiconductor laser device 62 is bonded onto the bottom of the concaveportion 61 a with solder, for example. A micro mirror 63 is provided ona sidewall of the concave portion 61 a so as to face the emissive endfacet of the semiconductor laser device 62 and to form an angle of 45degrees with the principal surface of the support member 61. In thisarrangement, the outgoing radiation 51 that has been emitted from thelaser chip 62 is reflected from the micro mirror 63 to advancesubstantially vertically to the principal surface of the support member61. In this case, the micro mirror 63 is preferably a (111)crystallographic plane of Si.

A monitoring photodiode 64 is formed on another sidewall of the concaveportion 61 a of the support member 61 so as to face the micro mirror 63.The photodiode 64 monitors the optical output power of the laser chip 62based on the laser radiation that has been emitted in a small quantityfrom the reflective end facet of the semiconductor laser device 62. Thesurface of the micro mirror 63 may be either Si or be coated with ametal thin film of Au, Ag or Al.

First and second photodiodes 65A and 65B are provided as photodetectorsfor receiving the light 52 that has been reflected from the optical disk(not shown). The photodiodes 65A and 65B are located in the upper partof the support member 61 so as to be parallel to the reflective plane ofthe micro mirror 63 and to interpose the micro mirror 63 therebetween.

As can be seen, the light-emitting and receiving sections are integratedtogether on a single support member of he optical integrated unit. Thus,this optical integrated unit can be smaller in size and thickness. Inaddition, since the semiconductor laser device 62 that can suppress theleakage of spontaneous emission to a minimum level is used as thelight-emitting section, the SNR at the light-receiving section can beincreased advantageously.

Embodiment 7

Hereinafter, a seventh embodiment of the present invention will bedescribed with reference to the accompanying drawings.

In the seventh embodiment, an assembly that can perform highly preciseautomatic power control (APC) on a semiconductor laser device will bedescribed.

Generally speaking, the optical output power of a semiconductor laserdevice changes depending on the current flowing through a laser chip.Thus, when a semiconductor laser device is applied to an optical diskapparatus or an optical information processor such as an opticalcommunications unit, the current flowing through the semiconductor laserdevice is regulated to obtain predetermined optical output power.Hereinafter, this relation between current and optical output power willbe described in further detail with reference to graphicrepresentations.

FIG. 16(a) illustrates an I-L characteristic between the operatingcurrent and optical output power of a semiconductor laser device, whileFIG. 16(b) illustrates how the I-L characteristics change depending onthe temperature. In FIG. 16(a), a variation ΔL in optical output poweragainst a variation ΔI in operating current is generally called “slopeefficiency”.

Suppose predetermined optical output power L₀ is attained when thetemperature and operating current are equal to T₁ and I₁, respectively,as shown in FIG. 16(b). In such a case, to maintain the optical outputpower at L₀, the operating current should be equal to I₂ and I₃ atsecond and third temperatures T₂ and T₃, respectively, where I1<I2<I3and T1<T2<T3.

The slope efficiency differs greatly among individual laser devices, andtherefore is very sensitive to the temperature. Specifically, as thetemperature rises, the threshold current tends to increase and the slopeefficiency tends to decrease as shown in FIG. 16(b). Accordingly, thehigher the temperature, the larger the operating current should be toattain predetermined optical output power.

Thus, in applying a semiconductor laser device with such temperaturedependence to an actual product, the operating current thereof should beregulated based on the temperature of the laser device to maintain thepredetermined optical output power L₀. Alternatively, the temperature ofthe laser device should be kept at a predetermined value.

An automatic power control technique for regulating the operatingcurrent of a semiconductor laser device by detecting part of laserradiation at a photoelectric transducer and monitoring the opticaloutput power is usually adopted as an alternative method for maintainingthe optical output power without depending on the temperature. Tomonitor the optical output power as required by the automatic powercontrol, the outgoing radiation emitted from a semiconductor laserdevice should be received at a photoelectric transducer such as aphotodiode. In general, laser radiation emits from both end facets of aresonant cavity mirror of a semiconductor laser device. Accordingly, theradiation emitted forward from one of these facets (hereinafter,referred to as “forward radiation”) is ordinarily allowed to go out of apackage, while the radiation emitted backward from the other facet(hereinafter, referred to as “backward radiation”) is usually receivedat a photodiode within the package.

In this case, non-directional spontaneous emission is radiated from theactive region of the semiconductor laser device toward surroundingregions before the current reaches a threshold value where laseroscillation starts. A component of the spontaneous emission that hasbeen radiated through the resonant cavity mirror of the semiconductorlaser device is represented in the I-L characteristic curve shown inFIG. 16(a) as optical output before the current applied reaches itsthreshold value.

Next, it will be described how the I-L characteristic varies with thematerial of the semiconductor laser device, i.e., depending on theoscillation wavelength.

FIG. 17 illustrates, in comparison, respective I-L characteristics ofInGaAlP and InGaAlN semiconductor laser devices with mutually differentoscillation wavelengths. In FIG. 17, the curve 5 represents the I-Lcharacteristic of the InGaAlP semiconductor laser device that is formedon a GaAs substrate and oscillates at a wavelength of about 650 nm. Onthe other hand, the curve 6 represents the I-L characteristic of theInGaAlN semiconductor laser device that is formed on a sapphiresubstrate and oscillates at a wavelength of about 400 nm. As can be seenfrom FIG. 17, the quantity of spontaneous emission radiated from theInGaAlN semiconductor laser device at a threshold value (curve 6) ismore than five times as large as that radiated from the InGaAlPsemiconductor laser device at the threshold value (curve 5). The reasonis believed to be as follows. Since the GaAs substrate absorbs light atwavelengths of 870 nm or less, the spontaneous emission that has beenradiated from the InGaAlP semiconductor laser device at the wavelengthof 650 nm is absorbed into the GaAs substrate. In contrast, the sapphiresubstrate cannot absorb the spontaneous emission that has been radiatedfrom the InGaAlN semiconductor laser device at the wavelength of 400 nm.

If a large quantity of spontaneous emission is radiated in this mannerbefore the current reaches its threshold value of oscillation, then thespontaneous emission is unintentionally input as bias light to themonitoring photodiode. Thus, the optical output power cannot becontrolled precisely even by the automatic power control technique.

Next, a package for a semiconductor laser device (hereinafter, simplyreferred to as a “semiconductor laser package”) according to the seventhembodiment of the present invention will be described with reference toFIG. 14.

FIG. 14 is a perspective view partly in section illustrating asemiconductor laser package 70 according to the seventh embodiment. Asshown in FIG. 14, the semiconductor laser package according to theseventh embodiment includes: a disklike base 71; a parallelepiped mount72; a submount 73; and a semiconductor laser chip 74. The base 71 andthe mount 72 are both made of a metal with excellent heat dissipationproperties. A side face of the mount 72 is secured to the upper surfaceof the base 71 such that the upper bottom of the mount 72 is parallel tothe axis of the base 71. The semiconductor laser chip 74 is bonded tothe upper bottom of the mount 72 with the submount 73 interposedtherebetween such that the optical axis of the laser radiation emittedtherefrom is substantially parallel to the axis of the base 71.

A photodiode 75, which function as a photoelectric transducer fordetecting the backward radiation Lb emitted from the semiconductor laserchip 74, is secured onto a region of the base 71 that faces thesemiconductor laser chip 74.

In addition, a light-blocking plate 76 with an opening 76 a is furtherprovided between the semiconductor laser chip 74 located over the upperbottom of the mount 72 and the photodiode 75 such that the optical axisof the backward radiation Lb passes through the opening 76 a. Thelight-blocking plate 76 may be made of any material not transmitting thelaser radiation, e.g., metal, plastic or glass. At least a surface ofthe light-blocking plate 76 that faces the semiconductor laser chip 74is preferably mat such that of the backward radiation Lb is notreflected from the surface of the plate 76 and combine with the forwardradiation Lf. Furthermore, the light-blocking plate 76 is preferablymade of a material absorbing the laser radiation or at least the surfaceof the plate 76 facing the laser chip 74 should be coated with a paint,for example, which absorbs the laser radiation.

A cap 77 is secured airtight onto the upper surface of the base 71 so asto cover the mount 72 and the light-blocking plate 76 entirely. A windowthat can transmit the forward radiation Lf is provided for the cap 77such that the optical axis of the forward radiation Lf passestherethrough. And the window is hermetically sealed with window glass78. Also, outer leads 79 for electrically connecting the semiconductorlaser chip 74 and the photodiode 75 to external components are providedon the lower surface of the base 71.

Laser radiation shows intense directivity and is emitted along theoptical axis thereof while expanding at a certain angle. In contrast,spontaneous emission is radiated from the semiconductor laser chip 74toward the regions surrounding the chip 74 in all the directions. Thus,it is usually difficult to separate a component of the spontaneousemission that has been emitted in the direction of the optical axis ofthe laser radiation from the laser radiation itself. However, accordingto this embodiment, the light-blocking plate 76 with the opening 76 a isprovided between the semiconductor laser chip 74 and the photodiode 75so as to allow the optical axis of the laser radiation to passtherethrough. In such a case, nothing but that component of thespontaneous emission in the direction of the optical axis of the laserradiation is detected by the photodiode 75.

As described above, in an InGaAlN laser device, which oscillates at awavelength of about 400 nm and includes sapphire, SiC or GaN substratefor the semiconductor laser chip 74, the substrate does not absorb thespontaneous emission. Thus, a larger quantity of spontaneous emissionleaks out of the semiconductor laser chip 74 compared to ared-light-emitting laser device with an oscillation wavelength of about650 nm. Accordingly, remarkable effects are attainable if thelight-blocking plate 76 is provided between the semiconductor laser chip74 and the monitoring photodiode 75 so as to attenuate the spontaneousemission passing through the plate 76 to a larger degree than thebackward radiation Lb passing through the plate 76 as is done in thisembodiment.

The larger the size of the opening 76 a, the lesser the degrees ofattenuation of the backward radiation Lb and spontaneous emission.Accordingly, the size should be set at an appropriate value such thatthe backward radiation Lb attenuates to a lesser degree and that thespontaneous emission attenuates to a larger degree. More preferably, thesize should be set at such a value as substantially eliminating theattenuation of the backward radiation Lb.

The shape of the opening 76 a of the light-blocking plate 76 may beeither circular or square. Specifically, the opening 76 a is preferablyoval so as to correspond to an ordinary beam spot shape of the laserradiation emitted from the semiconductor laser chip. However, the beamspot shape of the backward radiation might sometimes be irregulardepending on how the semiconductor laser chip has been mounted. Thus,the oval opening is not always an optimum one.

FIG. 15 schematically illustrates how to perform automatic power controlon the semiconductor laser package according to the seventh embodiment.In FIG. 15, the same components as those illustrated in FIG. 14 areidentified by the same reference numerals. As shown in FIG. 15, thebackward radiation Lb that has been emitted from the semiconductor laserchip 74 is photoelectrically converted by the photodiode 75 into anelectrical signal, which is then output as a monitor signal to an APCcircuit 80. In response to the monitor signal, the APC circuit 80regulates the operating current of the semiconductor laser chip 74 suchthat the optical output power of the semiconductor laser chip 74 is keptat a predetermined value.

According to the seventh embodiment, all of the spontaneous emissionradiated from the semiconductor laser chip 74, except for its componentemitted in the same direction as the backward radiation Lb of the laserradiation, is blocked by the light-blocking plate 76 provided betweenthe semiconductor laser chip 74 and the photodiode 75, and is notincident on the photodiode 75. Thus, the quantity of the spontaneousemission detected can be reduced to about one-tenth compared to asituation where the light-blocking plate 76 is not provided. As aresult, the adverse effects of the spontaneous emission can also bereduced even while the optical output power is still low, i.e., theoperating current is close to its threshold value, thus improving theprecision of the automatic power control.

In the prior art, the photodiode 75 unintentionally convertsphotoelectrically not only the backward radiation Lb detected but alsothat component of the spontaneous emission hat has been combined withthe backward radiation Lb in a relatively large quantity. Thus, thephotodiode 75 estimates erroneously the optical output power of thelaser chip higher than the actual one. As a result, the APC circuit 80selects operating current corresponding to optical output power lowerthan the predetermined one.

In the foregoing embodiment, the light-blocking plate 76 with theopening 76 a is used as an exemplary light-blocking means. However, theopening 76 a does not always have to be provided. For example, thelight-blocking plate 76 may be replaced with a thin film that attenuatesthe spontaneous emission to a larger degree than the laser radiation.

Also, the light-blocking means need not be a plate but may be acylindrical member extending in such a direction as allowing the opticalaxis of the backward radiation Lb to pass therethrough.

Furthermore, the light-blocking means is not limited to a memberprovided between the semiconductor laser chip 74 and the photodiode 75.For example, the light-blocking means may also be a gaseous or resinencapsulant that fills the inside of the cap 77 shown in FIG. 14 andthat attenuates the spontaneous emission to a larger degree than thelaser radiation.

It is naturally possible to reduce the spontaneous emission by providinga similar light-blocking plate 76 to the exemplified one for an InGaAlPlaser device of a long-wave-oscillating type, although the effectsattained are not so significant compared to the situation describedabove.

What is claimed is:
 1. A semiconductor laser device comprising: acontact layer, which is made of a nitride semiconductor of a firstconductivity type and is formed over a substrate; a first claddinglayer, which is made of a nitride semiconductor of a first conductivitytype and is formed over the contact layer; an active layer, which ismade of In_(y)Ga_(1−y)N and is formed over the first cladding layer; asecond cladding layer, which is made of still another nitridesemiconductor of a second conductivity type and is formed over theactive layer; and an In_(x)Ga_(1−x)N layer of the first conductivitytype is formed between the substrate and the first cladding layer,wherein 0<x<1, 0<y<1 and x≧y in the composition of In, and a substantialportion of light-emitting recombination occurs in the In_(x)Ga_(1−x)Nactive layer material.
 2. The device of claim 1, wherein theIn_(x)Ga_(1−x)N layer is formed in contact with the first claddinglayer.
 3. A semiconductor laser device comprising: a first claddinglayer, which is made of a nitride semiconductor of a first conductivitytype and is formed over a substrate; an active layer, which is made ofIn_(y)Ga_(1−y)N and is formed over the first cladding layer; a secondcladding layer, which is made of still another nitride semiconductor ofa second conductivity type and is formed over the active layer; anelectrode formed over the second cladding layer; and an In_(x)Ga_(1−x)Nlayer of the second conductivity type is formed between the secondcladding layer and the electrode, wherein 0<x<1, 0<y<1 and x≧y in thecomposition of In.
 4. The device of claim 3, wherein the In_(x)Ga_(1−x)Nlayer is formed in contact with the second cladding layer.
 5. The deviceof claim 3, wherein the In_(x)Ga_(1−x)N layer is formed in contact withthe electrode.
 6. An optical disk apparatus comprising the semiconductorlaser device of claim
 1. 7. An optical disk apparatus comprising thesemiconductor laser device of claim
 3. 8. An optical integrated unitcomprising a semiconductor laser device comprising: a contact layer,which is made of a nitride semiconductor of a first conductivity typeand is formed over a substrate; a first cladding layer, which is made ofa nitride semiconductor of a first conductivity type and is formed overthe contact layer; an active layer, which is made of In_(y)Ga_(1−y)N andis formed over the first cladding layer; a second cladding layer, whichis made of still another nitride semiconductor of a second conductivitytype and is formed over the active layer; and an In_(x)Ga_(1−x)N layerof the first conductivity type is formed between the substrate and thefirst cladding layer, wherein 0<x<1, 0<y<1 and x≧y in the composition ofIn, and a substantial portion of light-emitting recombination occurs inthe In_(x)Ga_(1−x)N active layer material.
 9. An optical integrated unitcomprising a semiconductor laser device comprising: a contact layer,which is made of a nitride semiconductor of a first conductivity typeand is formed over a substrate; a first cladding layer, which is made ofa nitride semiconductor of a first conductivity type and is formed overthe contact layer; an active layer, which is made of In_(y)Ga_(1−y)N andis formed over the first cladding layer; a second cladding layer, whichis made of still another nitride semiconductor of a second conductivitytype and is formed over the active layer; and an In_(x)Ga_(1−x)N layerof the first conductivity type is formed between the substrate and thefirst cladding layer, wherein 0<x<1, 0<y<1 and x≧y in the composition ofIn.